Um, no sir. Prequels have never been referred to as sequels. Not in publications, not in public discourse not even in your posts before this thread. Sequel doesn't mean "another installment" however you want to word that, sequel means follows the original in both narrative and production. That's why when someone says a sequel is coming, absolutely no one asks "Is it a prequel?" or "Is it a spinoff?" That's why the words prequel and spinoff were created, to say that they're not sequels.

And why is this the discussion? The OP was very explicitly in what he meant by #4 movie with lots and lots of examples. It's a legitimate question, even if you don't think it's important.